S-foci, the first reported mRNA-silencing foci specific to neurons, may control local mRNA translation in response to NMDA receptor stimulation and synaptic plasticity.

Repression of mRNA translation is linked to the formation of specific cytosolic foci such as stress granules and processing bodies, which store or degrade mRNAs. In neurons, synaptic activity regulates translation at the post-synapse and this is important for plasticity. Nmethyl-D-aspartate (NMDA) receptor stimulation downregulates translation, and we speculate that this is linked to the formation of unknown mRNA-silencing foci. Here, we show that the 59-39 exoribonuclease XRN1 forms discrete clusters associated with the post-synapse that are different from processing bodies or stress granules, and we named them synaptic XRN1 bodies (SX-bodies). Using primary neurons, we found that the SX-bodies respond to synapse stimulation and that their formation correlates inversely with the local translation rate. SX-bodies increase in size and number upon NMDA stimulation, and metabotropic glutamate receptor activation provokes SX-body dissolution, along with increased translation. The response is specific and the previously described Smaug1 foci and FMRP granules show a different response. Finally, XRN1 knockdown impairs the translational repression triggered by NMDA. Collectively, these observations support a role for the SX-bodies in the reversible masking and silencing of mRNAs at the synapse.

The production of proteins from mRNAs localized at the synapse ultimately controls the strength of synaptic transmission, thereby affecting behavior and cognitive functions. The regulated transcription, processing, and transport of mRNAs provide dynamic control of the dendritic transcriptome, which includes thousands of messengers encoding multiple cellular functions. Translation is locally modulated by synaptic activity through a complex network of RNA-binding proteins (RBPs) and various types of non-coding RNAs (ncRNAs) including BC-RNAs, microRNAs, piwi-interacting RNAs, and small interference RNAs. The RBPs FMRP and CPEB play a well-established role in synaptic translation, and additional regulatory factors are emerging. The mRNA repressors Smaug, Nanos, and Pumilio define a novel pathway for local translational control that affects dendritic branching and spines in both flies and mammals. Recent findings support a role for processing bodies and related synaptic mRNA-silencing foci (SyAS-foci) in the modulation of synaptic plasticity and memory formation. The SyAS-foci respond to different stimuli with changes in their integrity thus enabling regulated mRNA release followed by translation. CPEB, Pumilio, TDP-43, and FUS/TLS form multimers through low-complexity regions related to prion domains or polyQ expansions. The oligomerization of these repressor RBPs is mechanistically linked to the aggregation of abnormal proteins commonly associated with neurodegeneration. Here, we summarize the current knowledge on how specificity in mRNA translation is achieved through the concerted action of multiple pathways that involve regulatory ncRNAs and RBPs, the modification of translation factors, and mRNA-silencing foci dynamics.